The measurement of current flowing into and out of individual power outlets is a common feature of today's electrical wiring systems. This measurement may be accomplished using a current transformer. A current transformer is designed to produce either an alternating current or alternating voltage proportional to the current being measured. Referring to FIG. 1, a current transformer 100 common to the art is presented. Current transformers are often constructed by passing a single primary wire 101 (the primary turn) through a well-insulated toroidal core 102 wrapped with multiple turns of a transformer wire 103 (the secondary turns). The toroidal core 102 may be composed of ferrous materials such as iron, silicon steel, carbonyl iron or other such materials common in the art.
Current transformers are commonly used in metering and protective relaying in the electrical power industry where they facilitate the safe measurement of large currents, often in the presence of high voltages. The current transformer safely isolates measurement and control circuitry from the high voltages typically present on the circuit being measured.
The current transformer 100 utilizes the electromagnetic radiation emitted from the primary wire 101 as current flows in the wire 101. The electromagnetic radiation from the primary wire 101 induces a magnetic flux in the toroidal core 102. Based on Maxwell's Law, the current transformer 100 converts part of the electromagnetic field (EMF) produced by the primary current Ip 106 flowing in the primary wire 101 to a corresponding secondary transformer current It 107 in the transformer wire 103. The output of the current transformer may be placed across a load resistor 104 for measurement by a electrical measuring device 105. The accuracy of a current transformer is typically about 5%.
The current in the transformer wire 103 is in direct proportion (as specified by the number of turns in the transformer wire 103) to that of the primary wire 101. This relationship is governed by the Equation 1:
                              I          p                =                              I            t                    ⁡                      (                                          N                t                                            N                p                                      )                                              (                  Equation          ⁢                                          ⁢          1                )            where Ip is the current flowing through the primary wire 101, It is the current flowing through the transformer wire 103, Np is the number of turns in the primary wire 101 and Nt is the number of turns in the transformer wire 101. In most cases, the denominator Np may be disregarded as the number of turns in the primary wire is one (1) (i.e. the single pass of the primary wire through the current transformer). As such, once the current in the transformer wire 103 is measured via the measuring device 105, the current in the primary wire 101 may be calculated knowing the respective turns of the primary wire 101 and the transformer wire 103.
In power strip applications where many receptacles are to be monitored, multiple current transformers may be used. Referring to FIG. 2, a power strip wiring schematic 200 common to the art is presented. Power strips commonly incorporate numerous receptacles 201, 202 wired in series. The receptacles may be capable of receiving a connection cable for a given load such as a computer, lamp, or other electrical device. Current carrying live wires 203, 204, and 205 and neutral wires 206, 207, and 209 are connected to screw terminals 209 linking the respective receptacles together.
Current measurement for an individual power receptacle normally requires the use of two current transformers. A common method of monitoring the amount of current directed to a load connected to a power receptacle 201 is to place a current transformer 210 on an incoming live wire 203 and another current transformer 211 on the outgoing live wire 204 from the receptacle 201. The outputs of the current transformers 210 and 211 may be routed to separate load resistors 212 for conversion to an associated voltage. These resultant voltages may then be routed to amplifiers 213 to enhance the signals. The outputs of the amplifiers 213 may then be transmitted to subtraction circuitry 214. The resulting output value 215 corresponds to the amount of current directed to the load connected to the power receptacle 201. If the output value 215 should rise above a specified threshold value (indicating a short in a given load), the subject receptacle may be disabled to prevent damage to the circuit or harm to persons using the system.
Numerous drawbacks exist with respect to such a configuration. For example, two current transformers are required for each power receptacle. Also, each current transformer has a +/−5% accuracy. As such, the use of two current transformers for each receptacle results in a +/−10% accuracy in current measurement. Separate wiring for multiple amplifier and subtraction circuits may also be necessary thereby increasing complexity and cost of the system.
As such, it would be desirable to provide a current measuring system and method incorporating only one current transformer for each power receptacle to be monitored.